Carrier tracking loops are often used to demodulate signals received from satellites or from other rapidly moving bodies such as missiles, rockets and airplanes. The signals received from such sources are subject to comparatively large doppler shifts in the carrier frequency. For this reason, the carrier tracking loops must be able to acquire and to remain locked onto a signal having a variable frequency carrier. The natural pull-in time of a second order phase lock loop is well approximated by: ##EQU1## where .DELTA..omega. is the initial frequency offset between the loop center frequency and the incoming carrier frequency, .zeta. is the loop damping factor, and .omega..sub.n is the loop natural frequency. This approximation applies only where .DELTA..omega. is much greater than the loop bandwidth.
The acquisition performance indicated in equation (1) can be enhanced using a dual time constant integrator techinique, also called a variable bandwidth technique. In this approach, .omega..sub.n is large during signal acquisition, and is then reduced when the frequency error is reduced to some small value. This allows the loop signal to noise ratio to be improved during signal tracking. The dual time constant integrator scheme requires variable bandwidth filters, and offers only slight improvement in signal acquisition time, since making .omega..sub.n too large during acquisition results in too much reduction in the loop signal-to-noise ratio, and the loop misses acquisition completely. The dual time constant integrator scheme also requires a threshold detection circuit for determining the loop operating mode (acquisition or tracking), and for controlling the selection of loop bandwidth.
A second known method for acquisition time enhancement is to sweep the local oscillator frequency across the range of carrier frequency uncertainty. Although it is difficult to make general statements regarding the acquisition performance of swept local oscillator carrier tracking loops, the loop can acquire the signal with certainty only if the sweep rate D is less than 0.5 .omega..sub.n.sup.2. The approach is complicated by the circuitry needed to generate the periodic ramp waveform, and by the locking sensor for disabling the frequency sweep after acquisition occurs.
A third known method for acquisition time enhancement in carrier tracking loops uses a combined frequency discriminator-phase detector scheme. A number of different strategies are possible, such as using a phase detector and frequency detector, and adding these signals which in turn control the VCO, or using a phase frequency detector that produces an output voltage proportional to frequency when there is a frequency error, and when in lock produces a signal proportional to phase error. These acceleration techniques require relatively complicated circuitry and, in general, make it difficult to calculate acquisition time.